Data Encoding Techniques. Networks: ... The use of both analog and digital
transmissions for a computer .... Leon-Garcia & Widjaja: Communication
Networks.
Physical Layer – Part 2 Data Encoding Techniques
Networks: Data Encoding
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Analog and Digital Transmissions
Figure 2-23.The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs. Networks: Data Encoding
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Data Encoding Techniques • Digital Data, Analog Signals [modem] • Digital Data, Digital Signals [wired LAN] • Analog Data, Digital Signals [codec] – – – – –
Frequency Division Multiplexing (FDM) Wave Division Multiplexing (WDM) [fiber] Time Division Multiplexing (TDM) Pulse Code Modulation (PCM) [T1] Delta Modulation Networks: Data Encoding
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Digital Data, Analog Signals [Example – modem]
• Basis for analog signaling is a continuous, constant-frequency signal known as the carrier frequency. • Digital data is encoded by modulating one of the three characteristics of the carrier: amplitude, frequency, or phase or some combination of these. Networks: Data Encoding
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A binary signal
Amplitude modulation Frequency modulation
Phase modulation
Figure 2-24. Networks: Data Encoding
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Modems • All advanced modems use a combination of modulation techniques to transmit multiple bits per baud. • Multiple amplitude and multiple phase shifts are combined to transmit several bits per symbol. • QPSK (Quadrature Phase Shift Keying) uses multiple phase shifts per symbol. • Modems actually use Quadrature Amplitude Modulation (QAM). • These concepts are explained using constellation points where a point determines a specific amplitude and phase. Networks: Data Encoding
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Constellation Diagrams
(a) QPSK.
(b) QAM-16. Figure 2-25. Networks: Data Encoding
(c) QAM-64. 7
Digital Data, Digital Signals [the technique used in a number of LANs]
• Digital signal – is a sequence of discrete, discontinuous voltage pulses. • Bit duration :: the time it takes for the transmitter to emit the bit. • Issues – Bit timing – Recovery from signal – Noise immunity Networks: Data Encoding
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NRZ ( Non-Return-to-Zero) Codes Uses two different voltage levels (one positive and one negative) as the signal elements for the two binary digits. NRZ-L ( Non-Return-to-Zero-Level) The voltage is constant during the bit interval.
1 Ù negative voltage 0 Ù positive voltage NRZ-L is used for short distances between terminal and modem or terminal and computer. Networks: Data Encoding
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NRZ ( Non-Return-to-Zero) Codes NRZ-I ( Non-Return-to-Zero-Invert on ones) The voltage is constant during the bit interval. 1 Ù existence of a signal transition at the beginning of the bit time (either a low-to-high or a high-to-low transition) 0 Ù no signal transition at the beginning of the bit time
NRZI is a differential encoding scheme (i.e., the signal is decoded by comparing the polarity of adjacent signal elements.) Networks: Data Encoding
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Bi –Phase Codes Bi- phase codes – require at least one transition per bit time and may have as many as two transitions. Î the maximum modulation rate is twice that of NRZ Î greater transmission bandwidth is required. Advantages: Synchronization – with a predictable transition per bit time the receiver can “synch” on the transition [selfclocking]. No d.c. component Error detection – the absence of an expected transition can be used to detect errors. Networks: Data Encoding
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Manchester Encoding • There is always a mid-bit transition {which is used as a clocking mechanism}. • The direction of the mid-bit transition represents the digital data. 1 Ù low-to-high transition 0 Ù high-to-low transition
Textbooks disagree on this definition!!
Consequently, there may be a second transition at the beginning of the bit interval. Used in 802.3 baseband coaxial cable and CSMA/CD twisted pair. Networks: Data Encoding
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Differential Manchester Encoding • mid-bit transition is ONLY for clocking. 1 Ù absence of transition at the beginning of the bit interval 0 Ù presence of transition at the beginning of the bit interval
Differential Manchester is both differential and bi-phase. Note – the coding is the opposite convention from NRZI. Used in 802.5 (token ring) with twisted pair. * Modulation rate for Manchester and Differential Manchester is twice the data rate Î inefficient encoding for longdistance applications. Networks: Data Encoding
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Bi-Polar Encoding 1 Ù alternating +1/2 , -1/2 voltage 0 Ù 0 voltage
• Has the same issues as NRZI for a long string of 0’s. • A systemic problem with polar is the polarity can be backwards. Networks: Data Encoding
Analog Data, Digital Signals [Example – PCM (Pulse Code Modulation)]
The most common technique for using digital signals to encode analog data is PCM. Example: To transfer analog voice signals off a local loop to digital end office within the phone system, one uses a codec. Because voice data limited to frequencies below 4000 HZ, a codec makes 8000 samples/sec. (i.e., 125 microsec/sample). Networks: Data Encoding
Pulse Code Modulation (PCM) • Analog signal is sampled. • Converted to discrete-time continuousamplitude signal (Pulse Amplitude Modulation) • Pulses are quantized and assigned a digital value. – A 7-bit sample allows 128 quantizing levels.
Networks: Data Encoding
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Pulse Code Modulation (PCM) • PCM uses non-linear encoding, i.e., amplitude spacing of levels is non-linear. – There is a greater number of quantizing steps for low amplitude. – This reduces overall signal distortion.
• This introduces quantizing error (or noise). • PCM pulses are then encoded into a digital bit stream. • 8000 samples/sec x 7 bits/sample = 56 Kbps for a single voice channel. Networks: Data Encoding
Figure 2-33.T1 Carrier (1.544Mbps) Networks: Data Encoding
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Delta Modulation (DM) • The basic idea in delta modulation is to approximate the derivative of analog signal rather than its amplitude. • The analog data is approximated by a staircase function that moves up or down by one quantization level at each sampling time. Î output of DM is a single bit. • PCM preferred because of better SNR characteristics. Networks: Data Encoding
